Still-unobserved Decay
It is expected that some continual improvement of experimental sensitivity will allow discovery of very mild radioactivity (instability) of some isotopes that are considered to be stable today. For an example of a recent discovery, it was not until 2003 that bismuth-209 (the only naturally-occurring isotope of bismuth) was shown to be very mildly radioactive. However, there were also theoretical predictions from nuclear physics that bismuth-209 would decay very slowly by alpha emission. These calculations were confirmed by the experimental observations in 2003.
Many "stable" nuclides are possibly "metastable" in as much as they could be calculated to have a possible release of energy upon some very rare kinds of radioactive decay, including double-beta emission.
90 nuclides from the 40 elements with atomic numbers from one (hydrogen) through 40 are theoretically stable to any kind of nuclear decay -- except for the theoretical possibility of proton decay - which has never been observed despite extensive searches for it.
The nuclides starting from with the isotope niobium-93 and extending to all higher atomic mass numbers, could theoretically experience spontaneous fission.
For processes other than spontaneous fission, other theoretical decay routes for heavier elements include:
- alpha decay - 70 heavy nuclides (the lightest two are cerium-142 and neodymium-143)
- double beta decay (including electron-positron conversion, and double positron decay) - 55 nuclides
- beta decay - tantalum-180m
- electron capture - tellurium-123, tantalum-180m
- double electron capture
- isomeric transition - tantalum-180m
- cluster decay and spontaneous fission - the 56 heavy nuclides (from niobium-93 to dysprosium-164)
These include all nuclides of mass 165 and greater. Argon-36 is presently the lightest known "stable" nuclide which is theoretically unstable.
The positivity of energy release in these processes means that they are allowed kinematically (they do not violate the conservation of energy) and, thus, in principle, can occur. They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).
Read more about this topic: Stable Isotope
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